An automatic gain control, AGC, circuit varies the gain, or amplification, of a receiver so that its output remains at a desired intensity level irrespective of the magnitude of its input. In effect, an AGC produces an output regulated around a desirable threshold to obtain an optimal signal-to-noise ratio that is less dependent on the input signal level.
AGC circuits are routinely used in audio, video and other multimedia applications. For example, multi- channel receivers are typically designed for optimal performance within an ideal input range, but a channel's input strength will likely vary from channel to channel. An AGC inserted between the multi-channel receiver's input and its internal circuitry, will reduce the amount of signal strength variation from channel to channel and provide a more consistent signal strength to the receiver's internal circuitry. AGC circuits are also widely used as audio amplifiers that require their output power to be substantially independent of their input strength. Such audio amplifiers are used, for example, in dictaphones and codecs, which perform better if their audio amplifier's output is not greatly affected by the loudness of the person speaking into their microphone.
Thus, AGC circuits adjust the strength of an incoming signal to assure that it remains within a preferred operating range and thereby produce a more consistent output quality. Typically, AGC circuits do this by limiting the range of an input signal. This is done by reducing the intensity of the highest amplitude segments of an input signal and increasing the intensity of the weaker segments of the input signal. If desired, an AGC may also be used in expansion of an input by emphasizing the high amplitude segments and de-emphasizing the low amplitude segments of an input signal.
A general block diagram of a typical automatic gain control circuit 8 is shown in FIG. 1. An input signal Sin is applied to a gain correction unit 12, which adjusts its gain to amplify or attenuate Sin and produce an output signal Sout within a specified power range. Gain correction unit 12 adjusts its gain in response to a control input, CNTR, produced by a feedback system consisting of evaluation unit 10. Evaluation unit 10 receives both output signal Sout and an input reference threshold voltage. Evaluation unit 10 then compares a measure of Sout's intensity with the reference threshold voltage, and adjusts the gain of correction unit 10 as appropriate to maintain Sout within the intensity level dictated by the threshold voltage.
Evaluation unit 10 typically consists of a rectifier 14, an AC-to-DC converter 16, and a comparator 18. Sout is applied to rectifier 14 to remove negative components and thereby facilitate further processing of Sout. The output of rectifier 14 is applied to AC-to-DC converter 16, which also serves as a constant delay component to establish an "attack time", i.e. a wait period before altering the gain of correction unit 12. AC-to-DC converter 16 obtains a measure of the intensity of Sout by tracking its peak amplitudes, and its "attack time" slows down the response time of AGC 8 in order to reduce oscillation. If gain correction unit 12 were to have an immediate response to changes in Sout, then a slight spike in Sin might cause AGC 8 to continuously increase and decrease the gain of correction unit 12. In addition to the attack time, which is typically set from one to two milliseconds, comparator 18 is given a predetermined amount of hysteresis to further reduce oscillation.
Comparator 18 receives the output of from AC-to-DC converter 16 and also receives the reference threshold voltage. Comparator 18 does not alter the of gain correction unit 12 unless the output from AC-to-DC 16 drifts away from the reference threshold voltage by an amount beyond the comparator's hysteresis level. A typical automatic gain control 8 generally has a 5 dB to 10 dB range above the threshold voltage.
This circuit, however, has some limitations. For one thing, it does not provide much flexibility in establishing the "attack time" for various applications. Additionally, since a typical AGC tracks the intensity of its output signal Sout by monitoring its peak magnitudes, it is still prone to respond to temporary spikes and dips in Sout and degrade its overall output quality in spite of it "attack time" delay and its comparator's hysteresis.
Lastly, AGC 8 may actually reduce the quality of Sout under some circumstances. For example, if Sin experience a sudden increase, or decrease in amplitude while Sout is audible at a speaker output, not shown, then the corrective action of AGC 8 would result in corresponding sudden change in Sout. The sudden change in gain would likely cause a noticeable "clap" at the speaker output. This reduces the sound quality of the overall system.
It is an object of the present invention to provide an AGC that does not have a fixed delay for the application of both an increase gain command and a decrease gain command.
It is another object of the present invention to provide an AGC that is less likely to respond to short spikes in its input signal.
It is a further object of the present invention to provide an AGC that reduces the amount of noticeable clap at its output.